This invention relates to a method of fabricating high density silicon nitride bodies, and in particular to a method for removing densification aids from the grain boundaries of the silicon nitride bodies after hot pressing.
Because of scientific advances in fields requiring materials possessing good mechanical properties at high temperatures such as jet and internal combustion engines, rockets, atomic power generation, laser nozzles, and the like, the use of ceramics in the fabrication of structural components has been the subject of intensive research. Silicon nitride (Si.sub.3 N.sub.4) has been widely used in high temperature applications as a structural material for gas turbine blades and vanes because of its good high temperature strength and creep resistance, low thermal expansion coefficient, and excellent oxidation resistance.
However, because of poor sinterability, it is difficult to obtain a sintered body of silicon nitride having both high density and high mechanical strength. Two major methods have been used in attempting to fabricate silicon nitride structural components. In the first method, the silicon nitride is formed by reaction sintering silicon during a nitriding procedure. This method, however, generally fails to produce the necessary high density required because of porosity which remains in the structure during nitriding.
In the second method, various oxide fluxes or densification aids such as magnesium oxide, yttrium oxide, zirconium oxide, cerium oxide and other rare-earth oxides are added to silicon nitride powder before a hot-pressing procedure in which the silicon nitride body is subjected to heat and pressure in a mold. In such a hot-pressing procedure, bodies having densities approaching the theoretical density of silicon nitride can be fabricated. The oxide densification aids added to the silicon nitride react and produce glassy silicate compositions at grain boundaries. These glassy compositions have deleterious effects on the high temperature creep, slow crack growth, and strength properties of the silicon nitride bodies due to the softening or melting of the glassy compositions when exposed to temperatures of from 1000.degree. to 1200.degree. C., depending upon the particular densification aid used. These properties are important indicators of whether or not a part will fail during high temperature operation. In fact, poor elevated temperature slow crack growth properties of ceramics such as silicon nitride have been a major design barrier in their commercial use.
Consequently, most efforts to improve the high temperature properties of silicon nitride containing oxide densification additives have been directed toward improving the refractory character of the glass phase silicate compositions formed through use of composition control and crystallization methods. For example, Lumby et al, U.S. Pat. No. 3,989,782, teaches the use of a specific ratio of aluminum nitride, aluminum oxide, silicon dioxide, silicon nitride, and a combination of other metal oxides in a hot-pressing procedure which is said to result in the production of a sintered ceramic material (so called, sialon) having improved consistency, strength, and density. Ishii et al, U.S. Pat. No. 4,143,107, disclose a hot-pressing procedure which uses an oxide of a rare earth element and aluminum nitride in an attempt to avoid the formation of glass compositions. The procedure is designed to produce a densified silicon nitride body and to achieve crystallization of those substances which would otherwise produce glassy compositions having lower melting temperatures.
Likewise, Oda et al, U.S. Pat. No. 4,134,947, use specific ratios and weight percentages of beryllium, magnesium, strontium, and rare earth oxides as densification aids in a hot-pressing procedure to avoid the formation of glassy compositions. Finally, Mazdiyasni et al, U.S. Pat. No. 4,113,830, disclose a hot-pressing procedure utilizing rare earth oxides, hydrides, or nitrides which are taught to act both as densification aids and as scavengers for any impurities. These scavenger compounds are said to form, with any impurities, high temperature stable phases without affecting the physical and thermomechanical properties of the silicon nitride body. However, the patentees do acknowledge that under extreme use conditions (i.e., high temperatures) the presence of such compounds may have a deleterious effect on the hot-pressed silicon nitride body.
Accordingly, the need still exists in the art for a method of fabricating dense silicon nitride bodies which have improved elevated temperature slow crack growth and strength properties and which avoids the problems which have plagued the prior art.